In principle, any material of plant or animal origin, including textiles, wood, bones and leather, can be dated by its content of carbon 14, a radioactive form of carbon in the environment that is incorporated by all living things.Because it is radioactive, carbon 14 steadily decays into other substances.

Dating Subject to Error But scientists have long recognized that carbon dating is subject to error because of a variety of factors, including contamination by outside sources of carbon.Therefore they have sought ways to calibrate and correct the carbon dating method.The best gauge they have found is dendrochronology: the measurement of age by tree rings.Since 1947, scientists have reckoned the ages of many old objects by measuring the amounts of radioactive carbon they contain.New research shows, however, that some estimates based on carbon may have erred by thousands of years.

It is too soon to know whether the discovery will seriously upset the estimated dates of events like the arrival of human beings in the Western Hemisphere, scientists said.But it is already clear that the carbon method of dating will have to be recalibrated and corrected in some cases.Scientists at the Lamont-Doherty Geological Laboratory of Columbia University at Palisades, N.Y., reported today in the British journal Nature that some estimates of age based on carbon analyses were wrong by as much as 3,500 years.They arrived at this conclusion by comparing age estimates obtained using two different methods - analysis of radioactive carbon in a sample and determination of the ratio of uranium to thorium in the sample.In some cases, the latter ratio appears to be a much more accurate gauge of age than the customary method of carbon dating, the scientists said.